Radio Telescopes Provide Key Clue on Black Hole Growth

Astronomers have discovered the strongest evidence yet found
indicating that matter is being ejected by a medium-sized black
hole, providing valuable insight on a process that may have been
key to the development of larger black holes in the early
Universe. The scientists combined the power of all the
operational telescopes of the National Science Foundation's
National Radio Astronomy Observatory (NRAO) to peer deep into
the heart of the galaxy NGC 4395, 14 million
light-years from
Earth in the direction of the constellation Canes Venatici.

"We are seeing in this relatively nearby galaxy a process that
may have been responsible for building intermediate-mass black
holes into supermassive ones in the early Universe," said Joan
Wrobel, an NRAO scientist in Socorro, NM. Wrobel and Luis Ho of
the Observatories of the Carnegie Institution of Washington in
Pasadena, CA, presented their findings to the American Astronomical
Society's meeting in Seattle, WA.

Black holes are concentrations of matter so dense that not
even light can escape their powerful gravitational pull.

The black hole in NGC 4395 is about 400,000 times more massive
than the Sun. This puts it in a rarely-seen intermediate range
between the supermassive black holes at the cores of many galaxies,
which have masses millions to billions of times that of the Sun, and
stellar-mass black holes only a few times more massive than the
Sun. Energetic outflows of matter are common to both the
supermassive and the stellar-mass black holes, but the new
radio observations of NGC 4395 provided the first direct image
of such a suspected outflow from an intermediate-mass black hole.

The outflows presumably are generated by little-understood
processes involving a spinning disk of material being drawn
toward the black hole at the disk's center.

"An outflow from a black hole can regulate its growth by
pushing back on material being drawn toward it. This is an
important aspect of black hole development. Our observations
offer new and unique information on how this process works for
intermediate-mass black holes," Ho said.

"Intermediate-mass black holes may have been the starting
points for the supermassive black holes that we now see
throughout the Universe. By studying this contemporary analog
to those earlier objects, we hope to learn how the less-massive
ones grew into the more-massive ones," Wrobel explained.

The black hole in NGC 4395 was added to a small number of known
intermediate-mass black holes in 2005, when a research team led
by Brad Peterson of the Ohio State University calculated its mass
based on ultraviolet observations. Other ultraviolet and X-ray
observations gave tantalizing hints that material might be flowing
outward from the black hole.

"Fortunately, this object also is detectable by radio telescopes,
so we could use very high precision radio observing techniques to
make extremely detailed images," Wrobel said. Wrobel and Ho used
a technique called Very Long Baseline Interferometry (VLBI), in
which multiple radio-telescope antennas are used together to
simulate a much larger "virtual telescope," providing extremely
great resolving power, or ability to see fine detail.

The astronomers used all of NRAO's telescopes in their
coordinated VLBI array, including the continent-wide
Very Long
Baseline Array (VLBA), the 27-antenna
Very Large Array (VLA) in
New Mexico, and the giant
Robert C. Byrd Green Bank Telescope
(GBT) in West Virginia. The combination of antennas spread far
apart as well as the large amount of signal-collecting area in
this system allowed the scientists to make a detailed image of
the faint radio emission caused by fast-moving electrons in the
suspected outflow from the black hole interacting with magnetic
fields.

The resulting image showed the suspected outflow stretching
approximately one light-year from the black hole. "This direct
image bolsters the case for an outflow that was suggested by the
earlier indirect evidence from the ultraviolet and X-ray
observations," Wrobel said.

"By measuring the length of this suspected outflow, we offer
a unique constraint on theoretical models for how intermediate-mass
black holes operate," Ho said.